1. Field of the Invention
The present invention relates to optically variable devices allowing the propagation of light to be controlled and the optical quality of an image displayed by a unit comprising such a device to be improved.
More particularly the invention relates to an optically variable device comprising a liquid-crystal element having optical properties controlling the propagation of light, and two substrate plates placed on either side of the liquid-crystal element, the two substrate plates being covered with a first and a second control electrode, respectively, each electrode bounding a substantially central orifice called an optical aperture, the device furthermore comprising a layer of a material placed between the electrodes and filling at least the optical aperture.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
A device such as described above is known from document WO 2009/153764. More particularly, this document describes the use of a frequency-sensitive material, the conductivity of which changes when it is subjected to control signals of various frequencies. It is then possible to form a virtual lens in the liquid-crystal element, the focal length of which lens may be modified.
It is also known from the publication “Liquid-crystal adaptive lenses with modal control” [Naumov, et al.; Optics Letters, Vol. 23, Issue 13, pp. 992-994 (1998)] to use a weakly conducting material distributed in the optical aperture in order to control the voltage distribution between the edges and the center of the optical aperture by modulating the frequency of the control signals.
This type of lens, called an adaptive lens, has the advantage of having a variable focal length that can be rapidly changed using simple electrical means applying a voltage signal to the electrodes, the signal possibly having various frequencies. A converging lens is obtained the focal point of which lies along the optical axis.
Although this type of lens allows the focal distance along the optical axis to be varied as a function of the control voltage and frequency, it does not enable three-dimensional control of the focal point to be obtained allowing the direction of the optical beam to be shifted and thus the beam scan angular range of the device to be increased.